The present invention relates to a composite for use in forming multilayer printed circuit boards, and more particularly to a copper on INVAR(copyright) composite.
INVAR(copyright) (a registered trademark of Imphy S.A.) is a nickel-iron alloy containing about 36% nickel and 64% iron. It has been known to use copper/INVAR(copyright)/copper composite materials in the manufacture of printed circuit boards because of its low thermal coefficient of expansion (TCE). In this respect, copper/INVAR(copyright)/copper composite materials are commonly used in printed circuit boards as ground planes, power planes and/or metal cores, hybrid enclosures, heat sinks, and other areas where thermal coefficient of expansion matching is required or beneficial.
Heretofore, copper/INVAR(copyright)/copper composite materials were produced by metallurgically bonding two copper layers onto each side of an INVAR(copyright) layer. This mechanical process basically required rolling the respective layers together under pressure. A problem with metallurgically bonding a copper layer to an INVAR(copyright) layer is the inability of producing composite materials thinner than 2 mil. In addition, the metallurgical properties of the resulting copper/INVAR(copyright)/copper composites are not consistent throughout the resulting composite.
The present invention overcomes these and other problems and provides a copper on INVAR(copyright) composite that is thinner than copper/INVAR(copyright)/copper composites known heretofore, and that has more uniform consistent, metallurgical properties.
In accordance with a preferred embodiment of the present invention, there is provided a method of forming a copper on INVAR(copyright) composite comprising the steps of:
conveying a generally continuous strip of INVAR(copyright) along a path having a plurality of locations;
at a first location, cleaning the strip in an alkaline solution;
at a second location, cleaning the strip in an acid solution;
at a third location, applying a copper strike layer to the strip; and
at a fourth location, electrodepositing copper onto the strike layer.
In accordance with another aspect of the present invention, there is provided a composite for use in forming a multi-layer printed circuit board, comprised of an INVAR(copyright) sheet having a thickness of between 0.5 mil and 5 mil; and a layer of electrodeposited copper on at least one side thereof. The copper has a thickness of between 1xcexc and 50xcexc. The composite has a thermal coefficient of expansion (TCE) of about 2.8 to 6.0 ppm at temperatures between 0xc2x0 F. and 200xc2x0 F.
It is an object of the present invention to provide a copper on INVAR(copyright) composite that is thinner than similar composites formed by a metallurgical bonding process.
It is another object of the present invention to provide a composite as described above that has more consistent metallurgical properties than a metallurgically bonded composite.
It is another object of the present invention to provide a composite as described above that has a thermal coefficient of expansion of about 2.8 to 6.0 ppm at temperatures between 0xc2x0 F. and 200xc2x0 F.
These and other objects will become apparent from the following description of a preferred embodiment taken together with the accompanying drawings and the appended claims.